Method of treating unique martensitic alloys

ABSTRACT

A METHOD OF STABLIZING AN ALLOY FROM THE CLASS CONSISTING OF TINI,TICO, TIFE, TINIXCO1-X AND TICOXFE1-X BY ANNEALING IT AT 650* TO 700*C. AND COOLING IT SLOWLY TO A TEMPERATURE AT WHICH THE ALLOY DOES NOT UNDERGO THERMAL CYCLING.   D R A W I N G

METHOD oF TREATING UNIQUE MARTENSITIC ALLoYs Filed Feb. 26. 1968 July 20, 1971 FREDERICK E. WANG 2 Sheets-*Sheet 1 Gov wmnmmasw om o ow omkov om om o o ONI Om- Ov Om..

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Patented July 20, 1971 U.S. Cl. 148--13 6 Claims ABSTRACT or THE nrscLosURE A method of stabilizing an alloy from the class consisting of TiNi, TiCo, TiFe, 'I`iNixCo1 X and TiCoFe1 X by annealing it at 650 to 700 C. and cooling it slowly to a temperature at which the alloy does not undergo thermal cycling.

BACKGROUND OF THE INVENTION This invention relates to a new method of heat treatment of an alloy whereby its properties are improved in a controlled way. More particularly, the alloy is heated at 650 to 700 C.

Prior to this invention, alloys such as TiNi had been heated above its martensite temperature from about 600 C. to 850 C. for a time period slightly longer than that required to heat the material, which was usually less than an hour. The heating was in most cases followed by a cooling rate determined by the mass of the specimen and the normal heat flow to a still air surrounding. The treatment with near-stoichiometric TiNi varied slightly from this procedure in that the heating range was 700 C. to 800 C. for about a few hours.

It was noted the above described heat treatments caused variations in energy conversion efciency, mechanical memory and acoustic damping exhibited by the alloy. These variations previously had been attributed to slight alloy composition differences, extent of non-metallic inclusions (e.g., Ti4N2O, Ti4Ni2N, TiC, etc.) and bulk contamination through the transport of surface oxidation. However, the observed variations did not render the alloy useless but precise dimensional reproducibility was not hightly consistent or predictable. For example, a coiled wire or bent sheet of TiNi tended to get tired after repeated deforming and straightening by heating. This tiredness7 or relaxation is characteristic of the inability of TiNi to structurally recover. Furthermore, the amount of relaxation was found to vary from sample to sample.

The need for predictability in the properties of alloys lies in the electronics field where the specifications are very demanding and one must know precisely what the limits of the material are.

According to this invention, the variations are found to be related to the thermal cycling of the material, which can occur even at room temperature. In this way, fluctuations above and below certain critical temperatures cause energy to be stored in the alloy, which, as a result, does not exhibit the same value of resistivity or dimensional recovery among other properties as those which are Ineasured under identical temperature conditions. These deviations closely resemble the hysteresis eifect which is characteristic of alloys. However, for some unexplained reason, the alloy .exhibits better memory and damping characteristics as it is recycled several hundred times although these properties do reach a maximum value after which they decrease on exposure to further temperature cycles.

The variations in properties are found to be substantially eliminated by heating the alloy at 650 to 700 C. and then cooling it slowly to a temperature at which the alloy does not undergo thermal cycling. If the lower critical temperature is below room temperature, the alloy further may be provided with some mechanical stress but not enough to cause plastic deformation or cold working. Such deformation may be detected from a stress-strain curve and it is usually accompanied by slip, twining or dislocation movement. -In this way, it is possible to store the alloy at room temperature without it going through temperature cycles lwhich may have a degrading effect on its properties.

Another part of this invention relates to the determination of the critical temperatures at -which the properties of the alloy will not deteriorate. These temperature limits may be calculated by means of resistivity, resistance or damping measurements of which resistivity measurements are the most accurate. The upper critical temperature limit (TB) is the iirst temperature at which the resistivity of the alloy on cooling is equal to that resistivity obtained during heating for a given temperature as the alloy is cooled after being heated to above its martensite temperature. The lower critical temperature limit (TA), is the second point of equal resistivity for a given temperature obtained as the alloy is cooled. However, there is a practical problem in obtaining the lower limit, namely to insure intersection at the points of equal resistivity. But this ceases to be a problem because there is an observed sixty to seventy degree range in vwhich there occurs a point of equal resistivity. Therefore the alloy is cooled to approximately 60 to 70 C. below the observed TB temperature until this point is reached.

SUMMARY OF THE INVENTION Accordingly, an object of the instant invention is to provide a method of eliiciently converting heat energy of an alloy into mechanical energy.

Another object of this invention is to provide a method for minimizing the relaxation of alloys.

Another object of the invention is to provide a method of treatment whereby an alloy may be stored without degradation of any of its properties.

A further object of this invention is to provide an easy yand accurate method to determine the temperatures at which an alloy will not undergo thermal cycling.

Accordingly, these and other objects are obtained by heating an alloy above its martensite temperature and cooling it slowly to a temperature at which the alloy does not undergo thermal cycling.

BRIEF DESCRIPTION OF THE DRAWING A more complete appreciation of the invention and many of the attendant advantages thereof will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. l is a proposed phase diagram of TiNi showing the different types of TiNi which exist over specific temperature ranges;

FIG. 2 contains three plots of electrical resistivity against temperature for TiNi showing the effects of the thermal cycling; and

FIG. 3 contains additional plots of resistivity against temperature showing the effects of stressing T iNi.

DESCRIPTION OF THE PREFERRED EMBODIMENT The invention is more particularly set forth in the following example which is intended merely as a specific embodiment and is not to be construed as ya limitation of the scope thereof.

An example of an alloy which is within the scope of the invention is TiNi which exhibits the most desirable properties, such as energy conversion eiciency, me-

3 chanical memory and acoustic damping after it has been heated at 650 to 700 C. and then slowly cooled to allow it to pass through a martensite transition.

By means of crystallographic determination, such as X-ray diffraction, TiNi is found to have four distinct crystal structures as set forth in FIG. 1. By heating the alloy at 650 to 700 C., an ordering process takes place in the crystal structure and the imperfections due to thermal cycling are eliminated. However, care should be taken in heating the alloy that the temperature does not exceed 700 C. because then the alloy will contain some TiNi (I) at the martensite temperature (Ms) which in turn results in the formation of an undesirable mixture of TiNi states during the martensite reaction. The preferred temperature range is 650 to 700 C. for a period of about four days. However, lower temperatures may be used as long as they exceed the martensite temperature but then longer heating times would also be required. The martensite temperature for TiNi varies `and depends on the relative properties of Ti to Ni as disclosed in copending application Ser. No. 579,185, now abandoned, led on Sept. 9, 1966. For example, the martensite temperature for stoichiometric TiNi is about 170 C. Also, the heating may be done at either "6 mm, pressure or in the presence of a clean dry inert gas such as, for example, helium or argon in order to prevent oxidation and other interstitial contamination.

After the alloy is annealed and substantially all of it is in the TiNi (II) state, the alloy is slowly cooled below the martensite temperature whereupon it undergoes a martensite transition for the next 60 to 70 C.

This transition involves both electron and atom changes whereby there occur a localization and delocalization of electrons and also shear moments in a proper sequence and cooperative manner. Accordingly, resistivity measurements are made in order to determine the characteristics of the martensite transition.

A graph of resistivity versus temperature is plotted and a TiNi sample which has been heat treated in accordance with the invention yielded the plot described by FIG. 2(a). The triangle part of the resistivity curve is reproducible if the heating and cooling cycles proceed continuously in one direction until the temperatures (TA and TB) are exceeded before reversing the sample temperature direction. However, if the sample is cycled within TA and TB, the triangle becomes displaced and its area increases as shown by FIG. 2(b) which describes the result of a few heating cycles. The displacement and area of the triangle becomes exaggerated after several hundred of these cycles as indicated by FIG. 2(c). However, upon further thermal cycling the area decreases, thus exhibiting a maximum area for a given number of temperature cycles. The existence of the triangle within the 60 to 70 C. range is thought to be caused by a difference in the atomic shearing Buergers vector, the value of which depends on whether the alloy is heated or cooled. This vector is further affected if the alloy is cycled within this temperature range. In these measurements, the heating and cooling rates should not vary from test to test in order to produce the most meaningful comparisons.

The displacement and increase in the area of the triangle is significant in that it is accompanied by an improvement in the properties of the alloy. For example, experimentation has confirmed that there is a gain in the efficiency of converting heat energy into mechanical energy and also an improvement in memory for that alloy which exhibits the largest triangular area in a resistivity plot.

The sensitivity of the alloy within TA and TB range creates a practical problem of storing the alloy. Many of the near-stoichiometric TiNi alloys have a TB temperature of around 70 C. therefore the triangular area (0 to 70 C.) would include room temperature. Thus, storage of such material at room temperature (about 4 25 C.) will considerably affect its properties because of ordinary thermal fluctuations even though the material has been heat treated according to the invention.

This problem may be solved by storing the material either at 20 C. and lower or at 80 C. and higher so that ordinary liuctuations in temperature will not bring it within the triangular area. However, this solution is not altogether satisfactory because of the cost involved in keeping the temperature at these levels. Another more economical solution consists of mechanically stressing the material at a temperature below Ms but within its martensitic limit, that is, to such an extent that plastic deformation and work hardening will not occur. In this way, the heat treated alloy may be stored at room temperature without any degradation due to thermal cycling.

Some other effects of stressing are shown in FIG. 3 (a) through (e). By specic reference to the curves in FIG. 3(0), (d) and (e), it can be seen that not only the triangular area but also the temperature range from TA to TB is reduced. In addition, compression appears to be particularly elfective in reducing the area and the range.

The particular heating method and stressing treatment operates with other Cs-Cl-type alloys in addition to TiNi. For example. TiCo, TiFe and their intermediate ternary alloys of TiNixCo1 X and TiCoFe1 X wherein Ti denotes titanium and constitutes approximately 50 atomic percent of the composition, the term NiXC`o1 X denotes nickel and cobalt respectively and make up the remaining approximately 50 atomic percent of the composition, and the term CoxFe1 x denotes cobalt and iron respectively and make up the remaining approximately 50 atomic percent of the composition may be stabilized by the methods disclosed herein. In addition, alloys such as ZrPd, ZrRh, ZrRu and their ternary intermediates as well as HfPt, HfIr, HfOs and their intermediates are within the scope of the invention. These alloys are found to exhibit characteristics similar to TiNi as disclosed in copending U.S. patent application Ser. No. 579,185, now abandoned and therefore should also display a TiNi-type phase diagram.

Obviously, many modications and variations of the invention are possible in the light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specically described.

Having thus described the invention, what is claimed as new and desired to be secured by Letters Patent of the United States is:

1. A method of stabilizing an alloy selected from the group consisting of TiNixCo1 X and TiCoXFe1 X wherein Ti denotes titanium and constitutes approximately 50 atomic percent of the composition, the term NiCo1 x denotes nickel and cobalt respectively and make up the remaining approximately 50 atomic percent of the composition, and the term CoFe1 X denotes cobalt and iron respectively and make up the remaining approximately 50 atomic percent of the composition, comprising the steps of (a) annealing the alloy at 650 to 700 C. until said entire alloy attains the structure of TiNi (11); and

(b) cooling it slowly to a temperature below the martensitic temperature range at which the alloy does not undergo thermal cycling.

2. A method according to claim 1 wherein the alloy is TiNi.

3. A method of claim 1, which comprises the additional step of:

(c) subjecting said alloy after step (b) to a mechanical stress at a temperature which is below the martensitic temperature but within its martensitic limit which stress is less than the stress necessary to effect plastic deformation and work hardening.

4. A method according to claim 3 wherein the stress is compressive.

6 5. Method of improving the martensitic properties of an References Cited alloy yselected from the group consisting of TiNiXCo1 x and TiCoxFe1 x wherein UNITED STATES PATENTS Ti denotes titanium and constitutes approximately 50 3,174,851 3/1965 Buehlel' et al- 75-'170 atomic percent of the composition, the term 5 OTHER REFERENCES NxxCo1 x denotes nickel and cobalt respectlvely and Transactions of ASM v01. 55, 1962 pp. 269476.

make up the remaining approximately 50 atomic percent of the composition, and the term CoxFe1 X lgal Revew 161 126 No' 5 June 1 196:2 PP' denotes cobalt and iron respectively and make up the remaining approximately 50 atomic percent of the 10 lggurgllSgllfgdlgscs, V01- 36, NO- 10 October com osition which com rises:

(arg annealing the allly at 65.0 to 700 C. until Journal of Apphed Physlcs, vol. 34, No. 5, May 1963,

said entire alloy attains the structure of TiNi pp 14754477 (11); (b) cooling it slowly to a temperature below its 15 CHARLES N LOVELL Pnmary Examiner upper critical temperature limit TB; and U S Cl. XR

(c) thermal cycling said alloy between its upper critical temperature limit TB and its lower criti- 148-4, 16-7, 125, 133, 134 cal temperature limit TA until the alloy exhibits a maximum resistivity value. 2 6. The method of claim 5 wherein said alloy is TiNi. 

